MIG/MAG-welding is a welding process where an electrode is continuously fed toward a working piece. An electric power source generates a welding voltage and a welding current. During the welding process, the workpiece is heated primarily by an arc generated by the power source. The electrode is heated, partly by the power developed in the electrode as the weld current flows through an electrode stick out, and partly by the heat developed by the arc itself. The electrode stick out is a part of the welding wire between a free wire end and a contact tip, where the current transfer to the electrode takes place. A basic control of the welding process aims at achieving an electrode melting speed which corresponds to the electrode feed speed. Another basic control of the welding process is to enable the welding process to operate in a desired metal transfer mode. Further objects of the control may for instance be to influence the amount of heat transferred to the workpiece.
MIG/MAG-welding takes place in one of three basic metal transfer modes. In short arc welding, the material transport from the electrode to the workpiece takes place through short-circuiting droplets. A short arc welding process is schematically disclosed in FIG. 2. Since the process consists in alternating arc and short-circuiting droplet transitions, the average voltage between the electrode and the workpiece becomes low and thus the heat transfer to the base material will remain moderate.
When the supplied power is increased, one passes into the mixed arc area, where the material transport takes place through a mixture of short-circuiting and non-short-circuiting droplets. The result is an unstable arc which is difficult to control with a risk for much weld spatter and weld smoke. Welding in this area is normally avoided.
At a sufficiently high supplied power, the process enters the spray area, where the material transport takes place through small finely dispersed droplets without short circuits. The spatter quantity is clearly lower than in short arc welding. The heat supply to the base material will here be greater and the method is suitable primarily for thicker workpieces.
In the spray area pulsed welding is possible by use of an advanced controller controlling the power source. In pulsed welding the controller controls the wave shape of the welding current to ensure proper pinch off of the droplets one by one. Each pulse detaches a droplet and the droplets become sufficiently small not to short-circuit. This method results in advantages from the spray area in form of low weld spatter without the disadvantages of the large heat transfer.
A welding power supply may be described by its static and dynamic characteristics. The static characteristics of a power source describes how the output voltage is dependent on the output current with constant load conditions. The dynamic characteristics of a power source describes how the output voltage is dependent on the output current under varying load conditions.
The static characteristic of a welding energy source is frequently represented in a static voltage-current diagram (U-I diagram). The dynamic characteristic may be represented in diagrams of voltage against time and current against time or as voltage against current evaluated in time as working point movements.
Both static and dynamic characteristics of a welding energy source affect the welding process. As a result of the mutual interference between the static and dynamic characteristics optimization of the process is difficult.
Static characteristics of a power supply in a welding machine must be adapted to which metal transfer mode is selected for the welding process. A MIG/MAG-machine adapted for short arc welding is to be considered as a constant voltage source having a slightly decreasing characteristic, normally 3V per 100 A. This can be compared to a TIG welding machine where instead the current is constant.
In less sophisticated welding machines there is a setting knob for the electrode feed speed and a setting knob for the choice of one of several voltage outlets from the weld transformer in the welding machine. This may be replaced by a wheel for controlling the ignition angle on a thyristor for generating the weld voltage. In modern inverter machines, the weld voltage may be controlled with great precision. Modern inverter technology with switch mode power supply and micro processor controlled transistors offers faster and more precise control of both the static and the dynamic characteristics compared to other power supply configurations with thyristors or step controlled transformers that need to be adapted for each welding method and welding case.
To select a suitable reference value for the voltage for a particular electrode speed may be difficult for a welding operator, since an appropriate reference value is dependent on such factors as electrode material, electrode dimension and shielding gas type. In welding machines of today it is usual to include experience in form of suitable welding parameters for various electrode feed speeds for varying combinations of values of the influencing factors mentioned above, so called synergy lines, in the control computer of the welding machine. Producing such lines for all combinations of influencing factors represents an extensive work in the form of test weldings and documentation. In addition, the electrode quality may vary between different deliveries and thus lead to that previously tested synergy lines do not function any longer. Furthermore, shielding gases are now marketed with supplier specific names without specifying the composition of the gas. This also leads to problems in having a predetermined quantity of synergy lines suitable for all weld cases. Not even a later repetition of an apparently identical weld case is always successful since the composition of the gas or the weld electrode may have been changed by the manufacturer without notice. Obviously, this leads to a troublesome uncertainty when welding a new batch.